Replication of the human immunodeficiency virus HIV-1, the causative agent of acquired immune deficiency syndrome, (AIDS), requires virus budding from an infected cell maturation of the viral core through proteolysis, and fusion of virions with susceptible target cells. As the catalyst for HIV-1 maturation, the viral protease plays a key role in HIV-1 replication as evidenced by the success of protease inhibitors in AIDS therapy. HIV-1 maturation is a continuous process that initiates during budding from the cell and continues until formation of the viral core is complete. Because immature virions are defective for post-entry events essential for replication, a mechanism for preventing entry of immature virions into cells could be advantageous to the virus. Here we propose a novel mechanism for coupling HIV-1 entry into target cells with virion maturation. In this model, fusion of immature virions with target cells is inhibited through the binding of PR55Gag to the cytoplasmic domain of the transmembrane glycoprotein gp41. Upon cleavage of Pr55Gag by the viral protease, the structural constraints on the gp41 cytoplasmic domain are revealed, unlocking the fusogenic activity of the viral envelope proteins. Experiments will be performed to test this model. Using several assays of viral-cell fusion, the relative ability of mature and immature HIV-1 particles to enter susceptible target cells will be determined. The role of individual Pr55Gag cleavage sites in the regulation of HIV-1 entry will be analyzed by assaying the fusion activity of a panel of specific HIV-1 cleavage site mutants. The function of the gp41 cytoplasmic tail in the maturation-dependent regulation of HIV-1 entry will be analyzed by testing the fusion ability of immature virions containing a tailless gp41 protein. Conformational changes in the envelope protein complex that occur upon virion maturation will be analzyed using both biochemical and immunological approaches. These studies will reveal a novel mechanism for coupling HIV-1 entry with virion maturation, will contribute to our understanding of the effectiveness of HIV-1 protease inhibitors in treating persons with AIDS, and may facilitate the design of a desperately needed HIV-1 vaccine.